In general, a suspension is installed between a lower section of a vehicle body and a wheel axle to absorb and reduce vibration or shock transferred from wheels to the vehicle body. A suspension is provided with a spring and a shock absorber.
In such a suspension, a coil spring or an air spring may be used. In the case of an air suspension having an air spring, a compressor is driven by an engine or a motor, and air compressed by the compressor is stored in a main air tank through a water trap and a check valve. An internal pressure of the main air tank is adjusted to a predetermined range by a pressure regulator. A safety valve is installed at one side of the main air tank in preparation for abnormal air pressure. Air stored in the main air tank is supplied to an air spring of a suspension through an air pipe, a leveling valve, and a surge tank.
A conventional air spring generally has a spring constant that is set to be smaller than that of a coil spring. Accordingly, a natural frequency of a vehicle is lowered, which improves the ride comfort. However, if a spring constant is set to be too small, the vehicle steering stability is deteriorated. As such, there is a tradeoff between two characteristics, i.e., the ride comfort and the steering stability. That is, if one of the two characteristics becomes better, the other becomes worse.
However, a spring constant of a conventional air spring is fixed to a predetermined value. Therefore, if the ride comfort is improved, the steering stability is sacrificed. On the contrary, if the steering stability is improved, the ride comfort is sacrificed.